Process for oxidation of ammonia

ABSTRACT

Catalysts suitable for the oxidation of ammonia to nitrogen oxides which are suitable to produce nitric acid are prepared by doping cobalt oxide, preferably Co 3  O 4  with 0.1-10 atoms %, preferably 0.5-3 atom % of lithium. In a given case, there is added a mold assist agent to form tablets, spheres, extruded pressed strands or honeycombs. The product is calcined in air or an oxygen stream at 300°-700°C, preferably 400°-600°C and subsequently tempered at 700°-900°C in a reactive gas mixture of air and 8-10 vol. % ammonia to increase the form stability.

This is a division of application Ser. No. 282,044 filed Aug. 21, 1972, now U.S. Pat. No. 3,850,851.

The invention concerns a process for the production of a catalyst suitable to oxidize ammonia to nitrogen oxides which are used for the production of nitric acid. Among these nitrogen oxides are NO, NO₂ and N₂ O₃.

It is known that cobalt and its compounds with oxygen can be used as catalysts for the oxidation of ammonia to the above mentioned oxides of nitrogen. It is also known that catalysts which contain cobalt or oxygen compounds of cobalt produce high yields at average to high loads in the oxidation of ammonia to the nitrogen oxides. Additives, especially oxides of aluminum or bismuth act as favorable promotors. However, it is also known that the favorable effect of promotors such as aluminum oxide, for example, leads to a reduction of the activity of the catalyst if the catalyst is employed for the oxidation of ammonia under pressure.

Doping of cobalt oxide catalysts with potassium ions has been considered as favorable for the activity of the catalyst in the above mentioned reaction by several authors. (N. P. Kurin, P. E. Bogdanov: Kataliz Vyssh. Shkole, Min. Vyssh. Sredn. Spets. Tr. 1-go Mezhruz. Sovesch po Katalizu 2 (1958), 1 page 215).

However, it has been established that potassium doped cobalt oxide catalysts in operation have a loss of about 70% of the original potassium content within a short time. As a result, the advantage of doping is correspondingly lost.

Pure cobalt oxide catalysts, however, until now, have not been usable industrially since they sinter very strongly. For these reasons, there is proposed in German Offenlegungsschrift No. 2,033,134 the use of carrier materials whereby, however, the yields obtainable and load carrying ability likewise are greatly lowered.

The invention relates to the surprising observation that cobalt oxide catalysts which have been doped with a lithium ion concentration.sup. 1 after a short tempering time becomes form stable, have practically no more sintering and therefore shows an extraordinarily high catalytic activity and loadability in regard to the oxidation of ammonia to the above-mentioned nitrogen oxides. This type of catalyst consists of cobalt oxide, preferably Co₃ O₄ which contains 0.1 to 10 atom % of lithium, preferably 0.5 to 3 atom % lithium and which by appropriate shaping, as, for example, by pressing is converted to form tablets, spheres or honeycomb bodies, for example, and whose shape stability is guaranteed by a suitable tempering program.

While Co₃ O₄ is the preferred cobalt oxide, there can also be used CoO and Co₂ O₃. Any convenient source of lithium can be used which can be converted to lithium oxide in the treatment. Thus there can be used lithium hydroxide, lithium oxide, lithium nitrite, lithium nitrate or lithium salts of organic acids. If lithium compounds other than lithium hydroxide are used, equimolar parts of aqueous ammonia must be added.

The invention accordingly is directed to a process which comprises doping a cobalt oxide, preferably Co₃ O₄ with 0.1-10 atom %, preferably 0.5-3 atom % of lithium, thereupon, in a given case with the addition of a conventional mold assist agent such as graphite, other organic substances which can be burnt out, as for example, stearates, etc. in an amount of 0.1 to 3 % by weight forming into tablets, spheres, extruded strands or honeycombs, then calcining at a temperature of 300°-700°C, preferably 400°-600°C in air or an oxygen stream and finally tempering for form stabilization at 700°-900°C in a reactive gas mixture of air and 8-10 volume % ammonia.

The calcining is best carried out for 1-5 hours, preferably 2-3 hours. Besides it has also proven suitable to carry out the tempering treatment for 10-120 minutes, perferably 30-90 minutes, most preferably for about 60 minutes.

According to a preferred form of the process of the invention, the doped cobalt oxide is ground before mixing with the mold assist agent. However, the doped cobalt oxide can be ground together with the mold assist agent.

The doping with lithium can be accomplished in different ways. Proceeding as follows has proven especially suitable: After mixing an aqueous solution of a cobalt salt, e.g. cobalt nitrate, cobaltous chloride, cobaltic chloride, cobalt acetate, or cobaltic bromide with an aqueous solution of lithium hydroxide, the precipitate obtained is separated off, for example, by filtering or centrifuging, suspended in an aqueous solution of an oxidizing agent, as, for example, hydrogen peroxide, or weak organic peroxy acids such as peroxy acetic acid, after oxidation the solid material is separated off, for example, by evaporation of the suspension and after drying and shaping is first calcined and then tempered.

The oxidation of the suspended solid can be carried out at room temperature. However, it is preferred to accelerate the reaction by heating of the solid suspension, e.g. at a temperature up to 100°C. The end-point of the oxidation is shown when the development of gaseous O₂ ceases. Generally to attain this a heating to boiling of 20-40 minutes is sufficient.

The new catalysts are thus produced by mixing an aqueous solution of a cobalt salt such as cobaltous nitrate with a solution that contains lithium hydroxide. Thereby first precipitates out the basic compound of divalent cobalt which contains the necessary lithium doping. There is then separated off the precipitate from the remaining weakly alkaline aqueous solution, for example, by filtration or centrifuging and the solid material is suspended in an aqueous solution of an oxidizing agent such as hydrogen peroxide, for example, so that a reaction takes place between the solid material and the oxidizing agent. After the end of the reaction, the solid material is separated off again. This can be accomplished by evaporation of the suspension. The solid material is dried, mixed with graphite, pressed into suitably shaped bodies and these are calcined in the air or oxygen at a temperature of 300° to 700°C, preferably at 400° to 600°C. The molded article formed is finally tempered for 1/2 to 3 hours, preferably 1 hour, at a temperature of 700° to 900°C in an atmosphere which besides air contains 8 to 10 volume % of ammonia whereby the molded bodies slightly contract and acquire their final stabile form. The molded bodies produced in this manner possess a hardness and form stability comparable to a ceramic product and a very high catalytic activity in regard to the above-mentioned oxidation reaction. Besides its ability to carry loads does not abate in long continued operation.

The invention also includes the catalysts prepared by the described procedures which contain cobalt oxides, preferably Co₃ O₄ and 0.1-10 atom %, preferably 0.5-3 atom % of lithium with a structure imprinted by calcination in oxygen or air at 300°-700°C, preferably 400°-600°C and by subsequent tempering in a mixture of air and 8-10 volume % of ammonia at 700°-900°C.

A characteristic catalyst according to the invention and prepared in Example 1 in x-ray powder analysis showed d-value (A-units) and relative intensities I) as follows:d(A) I______________________________________4.70 middle2.87 strong2.464 weak2.440 very strong2.336 middle2.132 middle2.089 weak2.022 middle1.853 very weak1.650 middle1.555 strong1.505 weak1.427 strong1.365 weak1.276 weak1.232 middle______________________________________

Finally the invention includes the use of the catalysts for the oxidation of ammonia to such nitrogen oxides which can be used to produce nitric acid. The reaction can take place at 750° to 950°C using ammonia and air or oxygen. The ammonia can be 8 to 11 % of the gas mixture by volume.

EXAMPLE 1

700 grams of cobalt nitrate (cobaltous nitrate) were dissolved in 5 liters of water. These solutions were added to 2 liters of a solution which contained 150 grams of lithium hydroxide so that there were formed a precipitate and a weakly alkaline solution. The precipitate was filtered off and suspended in 40 liters of 10% aqueous hydrogen peroxide solution. The suspension was heated to boiling and evaporated to dryness. The solid residue was ground, mixed with 3% by weight of graphite and molded into tablets. The finished tablets were calcined for 3 hours at 600°C in an airstream and finally tempered for 1 hour at 800°C in an atmosphere which contained in addition to air 8.3 volume % of ammonia.

EXAMPLE 2

A catalyst produced according to Example 1 was inserted in a suitable reactor to a height of 6 cm, for example. The catalyst showed the following activities:

    Temperature                                                                           Gas Employed                                                                           GHSV  Trans-                                                                               Nitrogen                                                                            Specific load                                  (°C)                                                                           (Vol.% NH.sub.3,                                                                       (h.sup.-.sup.1)                                                                      formation                                                                            Oxide                                                                               (kg NH.sub.3 /m.sup.3 ×h)                       balance       %     Yield                                                      air)                (%)                                                 __________________________________________________________________________     785    10.0     50,000                                                                              99.9  98.7 3,500                                          800    8.7     100,000                                                                              99.8  98.0 6.200                                          820    9.5     135,000                                                                              98.8  96.5 9,100                                          830    8.3     200,000                                                                              96.6  95.0 11,800                                         __________________________________________________________________________

EXAMPLE 3

A catalyst produced according to Example 1 was inserted in a suitable reactor to a height of 6 cm and the transformation ascertained depending on the time at a GHSV (gas hourly space velocity) of 50,000 h .sup.⁻¹ employing a gas containing 10% ammonia, balance air. The same process was repeated with a catalyst doped with potassium ions by a known process and of equal volume and equal height of change. A comparison of the results with both catalysts is shown in the following comparison:

                               Nitrogen Oxide                                      Temperature                                                                               Duration                                                                            Transformation                                                                            Yield                                               Li+doped                                                                            + K doped  Li+ doped                                                                            K+doped                                                                             Li+ doped                                                                            K+doped                                       (°C)                                                                         (°C)                                                                          (hours)                                                                             (%)   (%)  (%)   (%)                                           __________________________________________________________________________     780  780   1    99.9  99.7 99.3  90.3                                          780  810   3    99.9  99.9 98.5  95.1                                          810  820   8    99.9  99.8 98.7  87.8                                          __________________________________________________________________________

EXAMPLE 4

Contraction, hardness (characterized by crushing strength), and alkali content of the catalyst produced by Example 1 were ascertained. For this purpose, the catalyst was operated at temperature of 780° to 830°C at space velocities up to 200,000 h.sup.⁻¹ operation was carried out with a mixture of 8 to 10 volume % ammonia, balance air. The results were as follows:Time of operation Hardness Contraction Lithium Content(h.sup.⁻¹) (kg/cm²) (%) (Atom %)______________________________________0 -- -- 0.721 -- 0.0 --8 -- 0.0 --40 150-200 0.0 0.73______________________________________

In an identical experiment with a catalyst produced by a known process which catalyst was doped with potassium, the potassium content in 5 hours was reduced from 0.3 to 0.04 atom % while the contraction amounted to 33%.

The catalyst of the invention therefore is well suited for the preparation of the nitrogen oxides such as NO, NO₂ and N₂ O₃ which are used to make nitric acid. 

We claim:
 1. A process for oxidizing ammonia to nitric oxides suitable for forming nitric acid comprising heating at 750° to 950°C a mixture of ammonia and air or oxygen in the presence of a catalyst said catalyst having been prepared by forming cobalt oxide doped with 0.1-10 atom % of lithium into shape, then calcining in air or oxygen at 300°-700°C and subsequently tempering at 700°-900°C in a reaction gas mixture of air and 8-10 volume % of ammonia to stabilize the form.
 2. A process according to claim 1 wherein the cobalt oxide is Co₃ O₄.
 3. A process according to claim 2 wherein the catalyst contains 0.5-3 atom % of lithium.
 4. A process according to claim 1 wherein the lithium doped cobalt oxide is prepared by mixing an aqueous solution of a cobalt salt and lithium hydroxide to form a precipitate, suspending the precipitate in an aqueous solution of an oxidizing agent and drying the suspension prior to calcining.
 5. A process according to claim 1 wherein the ammonia is oxidized at a temperature of 780° to 830°C. 